The present invention is a process for separating dimethyldichlorosilane from methyltrichlorosilane in a mixture. The process comprises contacting a mixture comprising methyltrichlorosilane and dimethyldichlorosilane with silica gel, where the dimethyldichlorosilane is selectively retained by the silica gel. The process provides for the recovery of a methyltrichlorosilane fraction reduced in dimethyldichlorosilane concentration. The present process is especially useful for removing low levels of dimethyldichlorosilane from methyltrichlorosilane.
The commercial production of methylchlorosilanes involves the contact of methyl chloride with silicon metalloid in the presence of a catalyst comprising copper at temperatures within a range of about 300.degree. C. to 350.degree. C. Typically this process is optimized for the production of dimethyldichlorosilane, with lessor amounts of methylsilanes, methylchlorosilanes, methylhydrosilanes, C.sub.2 to C.sub.7 hydrocarbons, polysilanes, polysiloxanes, silylmethylenes, and other species being formed. This product mixture usually undergoes a series of process steps such as distillation, condensation, and the like to effect separation and recovery of commercially important individual components of the product mixture. However, standard separation techniques based on the difference in the boiling point between compounds become difficult and expensive when the compounds have similar boiling points. This situation exist with the separation of methyltrichlorosilane (b.p. 66.1.degree. C.) and dimethyldichlorosilane (b.p. 70.1.degree. C.). The present inventors have found that silica gel can selectively remove dimethyldichlorosilane when in mixture with methyttrichlorosilane and therefore provide an alternative method for separation of these two methylchlorosilanes. The present process is particularly effective for removing trace amounts of dimethyldichlorosilane from methyltrichlorosilane, thereby providing methyltrichlorosilane essentially free of dimethyldichlorosilane contamination.
Wilkman et al., U.S. Pat. No. 5,290,342, describe a process for separating ethylsilane from silane by selective adsorption of the ethylsilane onto activated carbon.
Bothe et al., U.S. Pat. No. 5,445,742, describe a process for purification of halosilanes. The process consists of contacting a mixture comprising a halosilane and a hydrocarbon with an adsorbent selective for the hydrocarbon. Examples of useful adsorbents taught by Bothe et al. include activated carbon, carbon molecular sieves, and high silica zeolite.
The cited art does not recognize that silica gel can be used to selectively separate dimethyldichlorosilane from methyltrichlorosilane.